Communication systems in which information is transmitted from multiple sites are referred to as simulcast systems. In most simulcast systems it is desirable to maintain some level of synchronization of the transmitting units by the central controller. It is also desirable to transmit the information from the multiple sites as early as possible, in order to reduce delays affecting received signal quality. This reduction in quality arises both from user perceived degradation as well as degradations arising from delayed reflected energy from impedance mismatched four-to-two wire connections at target end terminals (e.g., wireline phones).
One such simulcast system in which transmission synchronization is critical is a cellular CDMA (Code Division Multiple Access) communication system incorporating soft-handoff. During soft-handoff, a subscriber receives voice or data transmissions from two or more base stations, or two or more sector transmitters, having overlapping coverage areas. Depending on the quality of the transmissions by the base stations, the subscriber may use all transmissions or choose the transmission of the base station having the best quality. However, where there is too much delay between the transmissions the subscriber is unable to perform the necessary diversity comparison of the transmissions for combination or to determine which has the best quality. This is made more critical since the communication (e.g., speech) is conveyed to the multiple sites in frames that must be assigned to fixed time windows of 20 msec for transmission. The use of such fixed time windows and synchronized timing allows the flames to be synchronously transmitted. However, a slight delay in the same frame arriving at one base station relative to another might cause the transmission at the one base station to be delayed one full time window relative to the transmission at the other base station(s).
This synchronization process is made inherently more difficult since base stations are almost always located such that transmission times from the central controller (e.g., a base site controller (BSC), mobile switching center (MSC) or the like) will differ. For accurate synchronization the difference in distance of the links or trunks connecting the base stations to the central controller should be taken into consideration. Typically this is accomplished by computing the delay from the central controller to all base stations and adjusting the transmission from the central controller based on the maximum possible delay, in other words the delay in transmission to the furthest served base station (likely including a significant "safety" margin). This results, however, in the information always being delayed by a maximal amount. Also, as the system expands and delays increase, further margins will have to be added to the built-in delay.
Another solution to these problems is disclosed in U.S. Pat. No. 5,268,933. There a finer degree of packet alignment resolution is achieved by delaying or advancing both packets based on a detection that either packet is greater than a predetermined size(.increment..sub.L), or is delayed beyond a predetermined time duration of a frame (e.g., 10 msec). While this broadly discloses a time alignment technique, it only specifically discloses a process using frame buffer monitoring, or time-stamped pointers, at the base stations; further, the vocoder responds to orders from both base stations. It does not specifically teach any method for minimizing timing adjustments, whereby PCM (pulse code modulated) discontinuities into a vocoder or like may be minimized, or insuring transmission of a packet in the right time window or air frame at the base station.
Accordingly, there exists a need for a system for transmission alignment in a simulcast system which provides a finer alignment resolution and insures simulcast transmission at each base station without requiring a timing source at the BSC or requiring undesirable adjustment levels at the central controller.